The oxidation states of intracellular and extracellular proteins are carefully managed by cellular redox machineries. Depuydt et al. discovered a reducing system that protects single cysteine residues from oxidation in the bacterial periplasm. DsbG, a thioredoxin-related protein, appears to be a key player in that system and is the first reductase identified in the periplasm of Escherichia coli. Together with DsbC, DsbG controls the global sulfenic acid content of this compartment. Sulfenic acid formation is a major posttranslational modification in the periplasm, and three homologous L,D-transpeptidases are substrates of DsbG. Sulfenic acid formation is not restricted to E. coli but is ubiquitous. Because proteins from the thioredoxin superfamily are widespread, similar thioredoxin-related proteins may control cellular sulfenic acid more widely.